This invention relates to a process for impregnating porous articles with a curable impregnant composition. More particularly, this invention relates to a process for heat curing impregnated porous articles which includes making successive incremental temperature and pressure increases, each of which are sustained during the curing process. The invention includes a heat curing system having a chamber for curing impregnated porous articles and, desirably, a heat transfer medium such as a liquid for immersing the impregnated porous part to be heat cured.
It is often desirable to form parts from lightweight metals in order to reduce the weight of a component or system and correspondingly reduce energy consumption as well as the costs of manufacture and maintenance thereof. With the advent of new machining technologies and emphasis on the environmental impact of power usage, more and more lightweight metals are being machined for more and more uses requiring these metals to perform multiple functions simultaneously. Examples of such metals include but are not limited to zinc, copper, brass, iron, aluminum, and various alloys. The terms xe2x80x9cporous partxe2x80x9d and xe2x80x9cporous articlexe2x80x9d are used synonymously herein to refer to components made from such metals.
An inherent problem with the use of lightweight metals is the presence of micropores which inhibit commercial viability. The occurrence of micropores is especially prevalent in components formed from metal powder. Porosity of porous parts is particularly problematic when such porous parts are utilized in fluid power systems or other liquid applications, where entrance of fluid in micropores can cause premature deterioration and fracture of the part. Other problems include the introduction of air and gas which may create processing or finishing difficulties as well as difficulties in the end use of the porous member.
In response to these problems, impregnation sealing technology emerged as a way to eliminate the micropores inherent in lightweight metal components yet retain the desirable performance characteristics thereof. During an impregnation sealing process, the porosity of porous articles is impregnated with a curable sealant composition, or xe2x80x9cimpregnantxe2x80x9d. Upon curing of the impregnant, the resulting sealed part is suitable for use in fluid exposure applications, as well as facilitating plating, coating and further processing of formed articles. The structural integrity of a porous part can also be enhanced through impregnation sealing. Sealing of porous parts maintains many advantages, including: rendering the parts leak-resistant or leak-proof; preventing or minimizing the incidence of internal corrosion in metal castings and sintered parts; increasing density to make the article capable of withstanding liquid or gas pressure during use; improving its strength; and preparing the surface of the article for a subsequent painting or plating operation.
The practice of using a liquid impregnant for the purpose of sealing the porosity of porous articles is a well-known and highly utilized process. Often an impregnation process is followed by an independent curing process. The curing process is conducted independently of the impregnation process to initiate and/or accelerate polymerization of an impregnant composition. Although cure can be accelerated by several factors, such as instantaneous temperature increases and removal of ambient air, the present disclosure is specifically concerned with heat curing processes and problems posed thereby.
A typical impregnation process is shown and described in U.S. Pat. Nos. 3,672,942, 4,416,921 and 5,273,662, all of which are incorporated by reference herein. To execute the conventional steps in the impregnation of a given part, the part is initially degreased and cleaned, then the cleaned part is subjected to vacuum aspiration in a vacuum tank, thereby attempting to remove entrapped air from the minute pores in the part. During immersion of the part in a bath of a curable liquid impregnant, such as an anaerobic or heat curable impregnant, the part is maintained in a vacuum. Subsequently, the immersed part is exposed to atmospheric pressure, thereby causing the liquid impregnant to permeate the minute pores of the part. Any residual liquid impregnant is returned to a storage reservoir and the part which has undergone the impregnation is centrifuged to expel any excess impregnant adhering to the surface thereof. Thereafter, the part is generally cleaned with detergent to remove liquid impregnant remaining on the surface of the part while leaving the impregnant within the pores.
The impregnated part is then conventionally subjected to a curing process, usually at elevated temperatures, to initiate and/or accelerate cure of the impregnant. This curing process is conventionally conducted at a standard temperature for the cure of the chosen impregnant. The impregnated part is placed in a curing chamber wherein curing temperature and pressure levels are pre-set in accordance with selection of part configuration, type of impregnant, end use of the part and other factors. The pre-set temperature and pressure levels are maintained for a time interval sufficient to initiate and achieve a desired level of cure, after which the parts are removed from the curing chamber and often subjected to post-cure treatments such as plating, painting and the like. Impregnation and curing processes are conventionally conducted in separate vessels, however, a common vessel may be employed during an impregnation-curing sequence.
Curing processes can and often are executed after completion of any of several types of impregnation processes. Conventional impregnation processes are accomplished generally by three methods: wet vacuum impregnation, wet vacuum/pressure impregnation or dry vacuum/pressure impregnation. Among these impregnation methods, wet vacuum impregnation techniques are generally employed more frequently than the dry vacuum/pressure method described herein. However, the steps required to complete each of these processes are similarly executed. To effectively illustrate the conventional impregnation processes, examples of such processes are schematically depicted in the flow diagrams of FIGS. 1 and 2. The numbers assigned to FIGS. 1 and 2 are indicative of the different operations or steps performed sequentially on a single containment vessel which is stationary.
During a conventional wet vacuum impregnation procedure (hereinafter xe2x80x9cWV processxe2x80x9d) as shown in FIG. 1, porous parts are placed in a single container or basket at Block 10. The parts and the vessel are then inserted into an impregnation chamber at Block 12 where both parts and basket remain stationary for the duration of the impregnation process. At Block 14, the parts are submerged into a vacuum tank substantially filled with a flowable sealant composition. While the parts are in the vacuum tank, a short term vacuum cycle removes air from the porosity of the parts at Block 16. The duration of the vacuum cycle is dependent upon the material characteristics of the part being treated and the type of sealant used as an impregnant. In this arrangement, the goal is to remove air from the pores of the part to allow impregnant to follow thereinto once the pressure is normalized to ambient pressure. The chamber is then returned to ambient pressure so that sealant penetrates the evacuated porosity of the parts. At Block 18, the parts may then be spun briefly in the basket to eliminate excess sealant from the part surfaces and to prevent undesirable surface curing of the impregnant thereon during the cure cycle.
The conventional wet vacuum/pressure impregnation process (hereinafter xe2x80x9cWVP processxe2x80x9d) has many common steps to the WV process shown in FIG. 1 with the difference being shown at Block 17 where the impregnation chamber is pressurized after the completion of the vacuum cycle at Block 16. Pressurization forces the sealant further into small porosity passages. The centrifuge step at Block 18 may then be carried out to remove and recover excess impregnant from the part surfaces and return the excess to a storage reservoir.
The conventional dry vacuum/pressure impregnation method (hereinafter xe2x80x9cDVP processxe2x80x9d) applies a vacuum to the porous parts before exposing the part to the impregnation sealant. This prior art method is shown in FIG. 2, wherein a basket of porous articles (not shown) is inserted into a containment chamber at Block 20 and placed directly in a dry vacuum chamber at Block 22. Air is evacuated at Block 24 from the porosity in the parts for a duration corresponding to several factors, among which are the type of part being processed and the type of sealant used as an impregnant. At Block 26 a transfer valve is opened from an impregnant storage reservoir which is in fluid communication with the impregnation chamber, allowing sealant to enter thereinto. The chamber is then positively pressurized beyond ambient pressure at Block 28 to force sealant into the parts. After impregnation, while residual sealant is being returned to the storage reservoir, a centrifuge operation carried out at Block 30 spins the porous articles to remove and recover excess impregnant.
Subsequent to impregnation of various porous articles, the articles are often subjected to a separate curing step in which polymerization of the impregnant is accelerated, often by an increase in temperature. A problem with curing of thermally-cured impregnation compositions is xe2x80x9cbleed outxe2x80x9d in which loss of resin from pores occurs due to expansion of residual air trapped in the heated porosity of the parts being sealed. When residual air is entrapped in the pores of the part, a full cure may be inhibited, leaving portions of the flowable impregnant uncured or only partially cured, and causing residual impregnant to migrate to the exterior surface of a porous part where it cures during the heat cure stage. See U.S. Pat. No. 3,900,940 for an example of how prior attempts have been made to address these drawbacks, albeit unsuccessfully.
When thermal energy is used to cure impregnant compositions in the porosity of the parts to be sealed, the impregnated parts may be heated to effect heat-curing of the composition with polymerization of the monomer component of the impregnant in the case of heat-cure compositions. Alternatively, in the case of anaerobic-cure compositions, the sealant may also be heated after impregnation has been effected to accelerate the anaerobic cure. (Meth)acrylic resins are often preferred due to their superior cure capabilities. However, the high rate of polymerization exhibited by such resins generates an extraordinary amount of heat due to the polymerization.
Impregnation sealants are usually applied in a vacuum/pressure combination process which consistently leaves a small quantity of air trapped in the interior of the part being sealed. Heating that trapped air can have two negative effects. First, the expansion of the gas causes sealant to flow out onto the surface of the part. Second, in parts with non-rigid structure, the gas expansion may actually cause structural failure of the component. In a state of the art process applied to graphite parts, for example, the parts are cured in air at 200xc2x0 F. The localized temperature in the porous part can reach as high as 400xc2x0 F. due to the considerable amount of exothermic energy that is released during bulk polymerization. This rapid and extensive expansion of trapped air can change the shape of the part or cause structural failure thereto.
The application of heat during a post-impregnation cure process exacerbates this problem by increasing the pressure of the gas which is trapped within the interior of the pores. As the temperature of the impregnant increases, the trapped gas expands. In weak substrates, expansion of impregnant-dwelling gas therein promotes structural damage thereto and subsequent performance failure thereof.
In addition, cured impregnant surface deposits may cause an impregnated article to vary from desired predetermined dimensional specifications, rendering the part useless for its intended function in applications requiring close dimensional tolerances. Furthermore, these unwanted surface deposits may interfere with subsequent painting, plating or assembly operations which frequently are performed on porous articles subsequent to impregnation. Moreover, removal of these deposits during such painting or plating operations results in contamination of the rinsing baths used in such operations and subsequent interference with adhesion of paint, plating or the like to the impregnated part.
The problems inherent in the above-described process may still occur when a constant temperature and/or pressure is applied. In addition, gas expansion is still likely to occur within the impregnated parts due to the combination of elevated temperature and constant pressure, resulting in internal structural damage to the articles. When the article is heat cured, the residue produces localized surface asperities which may interfere with subsequent operations or cause delamination of applied paint or plated films.
Thus, it is desirable to reduce the amount of bleed out experienced by an impregnant sealant composition and likewise control the application of temperature and pressure in an impregnation process so as to significantly reduce the occurrence of improperly impregnated parts. A need exists for a cure process, associated with an impregnation system, which addresses the aforementioned problems and overcomes them with minimal change to existing systems. The present invention fulfills this need.
It is an advantage of the present invention to provide a novel heat curing process for curing a polymerizable impregnant composition within a porous article impregnated therewith. The porous article is part of a system which includes a curing chamber which retains a heat transfer medium therein for accelerated curing of the impregnant composition. The heat transfer medium serves as a sink for exotherm during polymerization of the impregnant composition in a porous article. Examples of heat transfer media amenable to practice of the present invention include, without limitation, water, ethylene glycol and essentially any heat transfer medium or bath which is substantially unreactive to both the article and the impregnant composition.
It is a further advantage of the present invention to cure the impregnant composition by subjecting the impregnated article to successively discrete temperature and pressure increases within the curing chamber. Each successive temperature and pressure increment exceeds a previous temperature and pressure increment, respectively, and each successive temperature and pressure is sustained at a time interval sufficient to substantially normalize the temperature of the system. Escalation of temperature and pressure in this manner promotes cure of the impregnant composition until a predetermined desired temperature and pressure is obtained. The final desired temperature and pressure are maintained for a time sufficient to cure the impregnant composition to a desired extent. Although it is preferable to obtain full cure of the impregnant, a partial cure may be desirable in some instances, for example, when post-curing treatments are to be performed to prepare the article for its eventual application.
Selection of the temperature and pressure increments, as well as the appropriate time intervals, depends upon a multitude of factors. Such factors are chosen from selection criteria including type and amount of impregnant composition, configuration and size of said porous article, type of material used to fabricate said porous article, process of manufacturing said article, end-use of said porous article and combinations thereof. These criteria are provided for example only and do not limit the factors to be considered when determining adequate temperature and pressure increments in relation to time.
Thus, one embodiment of the present invention relates to a managed heat cure process for effecting improvement in the production of impregnated porous articles. As used herein, the term xe2x80x9cmanaged heat cure processxe2x80x9d denotes a novel post-impregnation heat curing process in which temperature and pressure is controllably increased and maintained in successively discrete levels while an impregnated porous article is submerged in a heat transfer medium in a curing chamber. In the present invention process, the temperature of any residual air inside of an impregnated article substantially equals that of the part itself. Thus, when a thermocouple is coupled to the article to measure the temperature thereof, it is assumed that the air within the part""s pores is also at the measured temperature.
Incremental temperature and pressure increases are controllably applied in relation to corresponding time intervals, wherein the duration of each interval is sufficient to promote cure of the impregnant composition. A cure process which utilizes such controlled increases in temperature and pressure significantly differs from prior art methods in which temperature and pressure are instantaneously set at predefined levels. In the present invention, as the air heats up and expands, pressure is accordingly increased in a stepwise fashion. The increasing pressure opposes the expanding volume of the heated residual air, resulting in fewer surface asperities and improving overall structural integrity of impregnated parts.
An additional embodiment of the present invention relates to a system for heat curing a polymerizable impregnant composition within a porous article impregnated therewith. The system includes a curing chamber having a heat transfer medium therein in which the porous article is placed. The system further includes means for controlling pressure and temperature in discrete successive increments wherein each successive temperature and pressure increment exceeds a previous temperature and pressure increment, respectively. During these successive incremental increases, each successive temperature and pressure is sustained at a time interval sufficient to substantially normalize the temperature of the system and also to promote cure of the impregnant composition until a desired maximum temperature and pressure is obtained. Means for verifying the discrete temperature and pressure increments may also be provided which enables maintenance of the maximum temperature and pressure to cure the impregnant composition. Like the aforedescribed method, the discrete temperature and pressure increments are specifically selected to minimize structural damage of said porous article.
A further embodiment of the present invention provides a porous article having porosity filled with a polymerizable impregnant composition and improved structural integrity and surface quality. This product is formed using the method and system described hereinabove.
An additional embodiment of the present invention provides a method of producing an improved impregnated porous article comprising the steps of: impregnating a porous article with a polymerizable impregnant composition; placing the impregnated article in a curing chamber having a heat transfer medium therein; heat curing said impregnant composition by subjecting said impregnated article to successively discrete temperature and pressure increases within said curing chamber wherein each successive temperature and pressure increment exceeds a previous temperature and pressure increment, respectively, and each successive temperature and pressure is sustained at a time interval sufficient to substantially normalize the temperature of said system and promote cure of said impregnant composition until a desired maximum temperature and pressure are obtained thereby. Maintaining said maximum temperature and pressure can be used to partially or fully cure said impregnant composition. The discrete temperature and pressure increments are desirably selected to minimize structural damage of said porous article. Determination of the temperature and pressure levels sufficient to cure the impregnant composition is made according to various criteria, such as those described hereinabove. Incorporation of characteristics that are endemic to the system enables optimization of the associated curing process to further alleviate the occurrence of structural damage to the parts during such cure process.